The present invention relates to semiconductor packaging and more particularly to a semiconductor device having solder wettable side-walls or ‘flanks’ to facilitate inspection of solder joints when the semiconductor device is mounted on a substrate or circuit board using a surface-mount technology (SMT) process.
Due to the manner in which semiconductor devices such as QFN (Quad Flat No lead) packages are singulated with a saw blade the surface of the exposed lead frame flank is flush with the molding compound such that solder does not readily climb-up or “wick” the flank or side-walls of the package meaning that the QFN package is not flank wettable.
FIG. 1 is a flow chart of a conventional assembly process for assembling a QFN device. The conventional process includes a wafer mount and saw step 10 followed by a die bond step 11 in which a die is bonded to a flag of a lead frame, and wire a bond step 12 in which the die is electrically connected to the leads of the lead frame. The lead frame typically is formed of copper and may be plated with another metal such as palladium (PPF) or silver (Ag). In the wire bond step die bond pads are connected to lead frame pads with bond wires. The bond wires may be gold, copper, aluminum or the like. This assembly is then encapsulated with a molding compound in step 13 followed by a laser marking and de-tape step 14. If a silver plated lead frame is used a further plating of tin or tin alloy (Sn) is applied in step 15. Finally, individual QFN devices are formed with a saw singulation step 16. The singulated QFN devices are then visually inspected and packed for shipment.
FIG. 2 is a partial cross-sectional view of a conventional QFN device assembled with a copper lead frame such as via the method described above. The QFN device is non-wettable at its flank due to the untreated copper surface 20 that is exposed yet flush with the side walls of the QFN device.
In view of the foregoing it would be desirable to have a QFN device that is solder wettable at its side walls or flanks to facilitate inspection of solder joints when the package is mounted such as to a printed circuit board (PCB) using an SMT process.